As easy-to-access and easy-to-produce hydrocarbon resources are depleted, there is an increased demand for more advanced recovery procedures. One such procedure is steam assisted gravity drainage (SAGD), a procedure that utilizes steam in conjunction with two spaced apart wellbores. Specifically, SAGD addresses the mobility problem of heavy oil in a formation through the injection of high pressure, high temperature steam into the formation. This high pressure, high temperature steam reduces the viscosity of the heavy oil in order to enhance extraction. The injection of steam into the formation occurs from a first wellbore (injector) that is drilled above and parallel to a second wellbore (producer). As the viscosity of the heavy oil in the formation around the first wellbore is reduced, the heavy oil drains into the lower second wellbore, from which the oil is extracted. Other advanced recovery procedures include thermal Assisted Gravity Drainage (TAGD), Toe to Heal Air Injection (THAI), Vaporized Hydrocarbon Solvent (VAPEX) production and Fire Flooding. In all of these advanced recovery procedures, the precise placement of two or more wellbores relative to one another is an important aspect of the process.
More specifically, the two wellbores are typically drilled at a fixed distance of only a few meters from one another. The placement of the injector wellbore needs to be achieved with very small margin in distance. If the injector wellbore is positioned too close to the producer wellbore, the producing wellbore would be exposed to direct communication of steam at very high pressure and temperature. If the injector wellbore is positioned too far from the producer wellbore, the efficiency of the particular advanced recovery process may be reduced. In order to assist in ensuring that the second wellbore is drilled and positioned as desired relative to the first wellbore, a survey of the two wellbores in the formation is often conducted. These surveying techniques are traditionally referred to as “ranging”. Based on ranging, the trajectory of the second wellbore, i.e., the wellbore being drilled, can be altered to ensure the fixed distance between the wellbores is maintained.
One common ranging technique employs electromagnetic (EM) systems and methods to determine direction and distance between two wellbores and ensure a desired distance between the wellbores is maintained. In typical EM ranging systems, an elongated, conductive pipe string, such as the wellbore casing, is disposed in one of the wellbores. This wellbore is often referred to as the “target” wellbore and usually represents the producer wellbore. In any event, a current is applied to this conductive pipe string in the target wellbore by a low-frequency current source. The current flows along the conductive pipe string and results in EM fields around the target wellbore. These EM fields around the target wellbore are measured using an electromagnetic field sensor system disposed in the other wellbore, which is typically the wellbore in the process of being drilled. This second wellbore usually represents the producer wellbore and the EM ranging device in the second wellbore is utilized to directly sense and measure the magnetic field from the target wellbore. The measured magnetic field can then be utilized to determine distance, direction and angle between two wellbores (a well pair), and if necessary, alter the trajectory of the wellbore being drilled. Ranging systems in which a current is injected into or gathered on the target wellbore in order to create or induce a magnetic field around the target wellbore are referred to as “active” ranging systems.
In some large scale drilling applications, a large number of wellbore pairs in close proximity to one another may extend in a formation, either drilled from individual drilling platforms or from a mother wellbore. In order to maximize production from the wellbore pairs and to avoid collision between wellbores, it is important to maintain the multiple wellbore pairs at a desired spacing from each other. Hence ranging techniques are not only needed between two wellbores of a particular system, but also needed between wellbore pairs, such as the injector wellbores between two adjacent wellbore pairs.